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Old 4th Jul 2020, 3:45 pm   #1
Slothie
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Default MK14 NIBL adapter

Well the subject of adapting the MK14 to run NIBL has been talked about in other threads and I've been thinking about it. At first I was considering the changes necessary to modify my series VI MK14 replica to support paged memory so I could create a board that plugged onto the edge connector and support the ROM and RAM needed for the NIBL-E interpreter, leaving page 0 unmolested for the standard memory and peripherals.
This approach however didn't appeal because it basically requires the redesign of most of the address decoding logic because SOC took shortcuts with the 'purity' of the design that worked but essentially didn't always use the NRDS and NWDS signals to Marshall the keyboard and display devices (for example). An alternative approach that I believe Timbucus first dug up in his research was to make a board that plugged onto the processor socket and intercepts the control and address lines as required. Another advantage of this approach is that the adapter would work on any MK14 not just my replica. Based on the article Tim found I sketched out the following circuit that latches the upper 4 address bits, and redirects access to pages 1-7 to on-board memory chips and pretends to the MK14 that a memory hole in page 0 is being accessed.
Luckily on most versions of the MK14 the NRDS and NWDS strobe the RAM IO chip so by intercepting those signals and forcing them high while changing the address lines to fall in the range decoded for the chip creates a reliable memory hole so when pages 1+ are addressed nothing on the MK14 main board responds. The links on the diagram set bits 8-11 to Axx with U4 which on the main board is the ram IO that won't respond because U3a,b force the main board and edge connector NRDS and NWDS signals high (U2a,b tristate the outputs in the case that the SC/MP is disabled by, for example the VDU). U5 is the latch that latches the upper address bits during the NADS cycle.
The components are listed as LS types but I can't see any reason why HC or HCT wouldn't work.
I must emphasise that this is completely untested, bit I hope in the next couple of weeks to be getting my Laptop and I'll be able to use my logic simulator to check things out because it's so easy to get things wrong!
I'm presenting this for comment. Things missing are some kind of on-board serial interface that takes into account the strange flow control requirements as is currently being discussed on other threads, and some way of arranging mass storage for those 24k BASIC programs!
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Old 4th Jul 2020, 5:43 pm   #2
Timbucus
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Default Re: MK14 NIBL adapter

This is great work lets hope you can get your gear and progress this - I will help where I can of course as it has been one of my goals. Since I was unable to share my copy of the article due to it being a licensed reprint from the Museum someone has scanned the issue to archive.org so you can see it here:

https://archive.org/details/Personal...ge/62/mode/2up

One thought it might be nice to support an 8K ROM though to allow Karen O's Page 2 addon on-board Assembler.

https://www.vintage-radio.net/forum/...d.php?t=168405

I have been toying with an i2c interface for something like a 24LC256 on small removable cards so with some code squeezed into the Page 2 ROM you would have a relatively cheap mass storage device that should be able to be used fairly easily.
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Old 4th Jul 2020, 8:19 pm   #3
Mark1960
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Default Re: MK14 NIBL adapter

I知 taking a slightly different approach, designing a pcb for RC2014 to fit an sc/mpII. This would then allow use with a range of existing modules.

I think it could share the bus with a z80, so I can use one of my existing modules to load code to ram, then halt the z80 and enable the sc/mpII.

As I couldn稚 find a transparent latch with tristate output, I知 going to try a 74hct257. This also avoids inverting NADS.

Also want to experiment with multiprocessing two sc/mpII.

I値l add a schematic when I get a bit further, if anyone is interested in reviewing for mistakes.
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Old 4th Jul 2020, 8:45 pm   #4
Timbucus
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Default Re: MK14 NIBL adapter

Quote:
Originally Posted by Mark1960 View Post
I知 taking a slightly different approach, designing a pcb for RC2014 to fit an sc/mpII. This would then allow use with a range of existing modules.

I think it could share the bus with a z80, so I can use one of my existing modules to load code to ram, then halt the z80 and enable the sc/mpII.

As I couldn稚 find a transparent latch with tristate output, I知 going to try a 74hct257. This also avoids inverting NADS.

Also want to experiment with multiprocessing two sc/mpII.

I値l add a schematic when I get a bit further, if anyone is interested in reviewing for mistakes.
Sounds an interesting project that joins two of my favourite chips maybe we can start a new thread over in Homebrew for it could you not use the 81LS95 - they seem fairly plentiful on the second hand market and are contemporary with the SC/MP designs?
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Old 4th Jul 2020, 9:08 pm   #5
Mark1960
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Default Re: MK14 NIBL adapter

Quote:
Originally Posted by Timbucus View Post
Sounds an interesting project that joins two of my favourite chips maybe we can start a new thread over in Homebrew for it could you not use the 81LS95 - they seem fairly plentiful on the second hand market and are contemporary with the SC/MP designs?
I don稚 think I know the Homebrew forum, I値l try searching.

I知 planning to use the hct257 as a latch, similar to the 157 in the mk14 display circuit, but with tristate output controlled by nenin of the first sc/mpII. Not sure if the 81ls95 could easily be used as a latch.
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Old 4th Jul 2020, 9:17 pm   #6
Timbucus
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Default Re: MK14 NIBL adapter

Quote:
Originally Posted by Mark1960 View Post
Quote:
Originally Posted by Timbucus View Post
Sounds an interesting project that joins two of my favourite chips maybe we can start a new thread over in Homebrew for it could you not use the 81LS95 - they seem fairly plentiful on the second hand market and are contemporary with the SC/MP designs?
I don稚 think I know the Homebrew forum, I値l try searching.

I知 planning to use the hct257 as a latch, similar to the 157 in the mk14 display circuit, but with tristate output controlled by nenin of the first sc/mpII. Not sure if the 81ls95 could easily be used as a latch.
No it can't it is just a transparent buffer my bad - I just meant the bit of this forum for not vintage computers which Karen hangs around in and publishes these sorts of things...

https://www.vintage-radio.net/forum/...splay.php?f=99
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Old 4th Jul 2020, 9:44 pm   #7
Slothie
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Default Re: MK14 NIBL adapter

The address latch doesnt need to be transparent as the data bus is stable on the rising edge of NADS so you can use a 74x173 which is positive edge triggered latch without needing to invert it. Using a multiplexer as a latch is not reliable according to some people as was discussed in the early parts of the mk14 schematics thread.
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Old 4th Jul 2020, 10:59 pm   #8
Mark1960
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Default Re: MK14 NIBL adapter

Quote:
Originally Posted by Slothie View Post
The address latch doesnt need to be transparent as the data bus is stable on the rising edge of NADS so you can use a 74x173 which is positive edge triggered latch without needing to invert it. Using a multiplexer as a latch is not reliable according to some people as was discussed in the early parts of the mk14 schematics thread.
I started with 74hct173, then decided to switch to the 74hct257 as a transparent latch instead. I was thinking about extending the time for stable address set up, but then realised the timing of the scmp bus is so slow that it really doesn稚 matter, but then I was just amused to misuse the multiplexer. I値l take a look through those early mk14 posts.

Anyway sorry to divert the topic of this thread.
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Old 5th Jul 2020, 8:24 am   #9
Slothie
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Default Re: MK14 NIBL adapter

Not at all, often important and interesting things come from the tangents, as long as their vaguely related
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